1. Field of the Invention
The invention relates to internal combustion engines, and more particularly, to a fuel management system for an internal combustion engine fueled by a liquid hydrocarbon.
2. Description of Related Art
The operation of internal combustion engines is well known. In an internal combustion engine, combustion of fuel takes place in a confined space, producing expanding gases that are used to provide mechanical power. The most common internalcombustion engine is the four-stroke reciprocating engine used in automobiles. Here, mechanical power is supplied by a piston fitting inside a cylinder. On a downstroke of the piston, the first stroke, fuel that has been mixed with air (by fuel injection or using a carburetor) enters the cylinder through an intake valve via an intake manifold. The intake manifold is a system of passages that conduct the fuel mixture to the intake valves. The piston moves up to compress the mixture at the second stroke. At ignition, the third stroke, a spark from a spark plug ignites the mixture, forcing the piston down. In the exhaust stroke, an exhaust valve opens to vent the burned gas as the piston moves up. A rod connects the piston to a crankshaft. The reciprocating (up and down) movements of the piston rotate the crankshaft, which is connected by gearing to the drive wheels of the automobile.
A diesel engine is another type of internal-combustion engine. It is generally heavier and more powerful than the gasoline engine and burns diesel fuel instead of gasoline. It differs from the gasoline engine in that, among other things, the ignition of fuel is caused by compression of air in its cylinders instead of by a spark. The speed and power of the diesel are controlled by varying the amount of fuel injected into the cylinder.
In this disclosure, a fuel is defined as a substance that can be burned by supplying air and a sufficient amount of heat to initiate combustion. A liquid hydrocarbon fuel, such as gasoline or diesel fuel, must be converted to a gas before it can be ignited. This liquid to gas vapor conversion is required because the molecules of fuel must be well mixed with the molecules of air before they can chemically react with each other to give off heat.
However, not all of the liquid fuel must be converted to a gas before combustion can occur. Just enough fuel needs to be converted to a gas so that the mixture of gas molecules and air molecules falls within the fuel's flammability limits--which refers to the minimum and maximum concentration percentages, by weight, of fuel in air that will burn. If the concentration of the gaseous fuel in air is less than the minimum or greater than the maximum flammability limit, the fuel and air mixture will not ignite. Known internal combustion engines and fuel delivery systems are inefficient in converting the liquid fuel to a gaseous state. Therefore, the fuel and air molecules cannot mix properly for complete combustion.
In a gasoline engine employing a standard automotive throttle body fuel injection system, this inefficiency is due at least in part to the high velocity of the air and fuel mixture passing the fuel injection's throttle body, which may reduce the inlet temperature as low as 40.degree. F. (4.degree. C.). The flash point temperature--the temperature at which the fuel will give off enough vapor to form a combustible mixture with air--for gasoline is 45.degree. F. (7.degree. C.). This reduction in inlet temperature reduces the amount of heat available from the atmosphere to evaporate the fuel. Since less ambient heat is available, more energy from compressing the mixture is required to evaporate the fuel.
Gasoline engines have a throttle valve to control the volume of intake air. The amount of fuel and air that goes into the combustion chamber regulates the engine speed is and, therefore, engine power. This causes continuous changes in the atmospheric air velocity due to the pressure differential between the atmosphere and the intake manifold. These pressure variations cause the size of the particles of atomized fuel to vary throughout the engine's RPM range. As a result, there is a wide variation in fuel droplet size in the air stream. Therefore, the fuel droplets have less surface area exposed to the air for evaporation and more heat is required to fully evaporate the fuel.
Once the fuel vapor and air mixture leaves the throttle body injector and enters the intake manifold, the mixture velocity is so high that some of the fuel droplets are centrifuged out of the air stream when they make turns. This occurs because the fuel droplets are heavier than air. This varies that portion of the mixture's stoichiometric fuel to air ratio, even though the overall air to fuel ratio of the mixture flowing through the fuel injector is correct. The portion of the mixture that contains the fuel that was centrifuged out of the main air stream reduces the amount of surface area exposed by the fuel to the air for evaporation. This increases the amount of energy required to evaporate it. Once this portion of the fuel mixture is evaporated, it bums rich since the original portion of this mixture was rich from the fuel being centrifuged out of the main air stream. Carbony residues that accumulate in the combustion chambers and darker areas on the piston tops indicate areas of excessive fuel richness during combustion.
Conversely, portions of the air stream that are lean, but still fall within the flammability limits, will bum and cause extremely high temperatures. Auto-ignition temperature refers to the temperature at which a mixture of air and fuel will spontaneously ignite without open flame, spark, or a hot spot. The auto-ignition temperature of gasoline is 495.degree. F. (275.degree. C.). When these localized high temperature areas reach high enough pressure and temperature, autoignition of the end gases will result, causing detonation, which is the uncontrolled combustion or explosion caused by autoignition of the end gases that were not consumed in the normal flame front reaction. Detonation results in the familiar "ping" or "spark knock" sound.
The engine's heat of compression during the compression stroke produces heat that begins to evaporate the air and fuel mixture in the cylinder. However, this compressing of the mixture increases the pressure. As a result, the increased pressure increases the boiling point of the fuel for evaporation. Evaporation continues slowly because these relationships are not linear. So enough fuel evaporates, allowing it to fall within its flammability limits. Then the spark plug ignites the mixture and creates a flame front. This flame front during the combustion process has the same effect of increasing the boiling point of the fuel so its critical temperature is never reached. Therefore, the remaining atomized fuel droplets do not evaporate before or during combustion. Since the droplets are not vaporized, they do not burn.
When the cylinder pressure falls due to the descent of the piston while on the power stroke, the fuel droplets that were not evaporated earlier now evaporate due to a lower boiling point and higher cylinder temperature. These evaporated fuel droplets now burn, but they burn too late into the crankshaft angle for producing power. Thus, less is power and high exhaust gas temperatures result.
Direct (intake) port fuel injection has better fuel distribution characteristics than a throttle body fuel injection system. However, they allow very little time to evaporate fuel in the intake port. Therefore, the heat of compression must heat the air/fuel mixture for evaporation before combustion can occur. This system has the same inherent inefficiencies regarding the engine's heat of compression, which increases the boiling point of the fuel. Therefore, as the cylinder pressure rises, the critical temperature is never reached. The remaining fuel droplets do not burn in time to produce power. Thus, less power and high exhaust gas temperatures still result.
The heat of combustion (the temperature in the cylinder due to combustion) for gasoline is 840.degree. F. (449.degree. C.) plus or minus 40.degree. F. (4.degree. C.) above ambient. Conventional automotive exhaust gas temperatures are 1,400 to 1,500.degree. F. (760 to 815.degree. C.). This temperature difference (heat energy) between the exhaust gas temperature and the heat of combustion is totally wasted as excessive exhaust gas temperature. Even the engine's cooling system must be enlarged to dissipate the higher exhaust gas temperatures due to the increased temperature differential around the exhaust side of the combustion chambers and exhaust ports. This wasted heat energy is dissipated to the atmosphere through the vehicle's radiator, and an equal amount of wasted heat energy is dissipated through the vehicle's exhaust pipes as excessively high exhaust gas temperatures.
The remaining fuel that did not chemically react in the combustion chamber or in the exhaust manifold then enters a 2,000.degree. F. (1,093.degree. C.) catalytic converter for combustion. The unburned fuel that escapes the catalytic converter enters the atmosphere as hydrocarbon and carbon-monoxide pollutants. Moreover, currently produced catalytic converters are only effective when the engine is at operating temperature, so it has no effect on cold start emission levels.
Similar shortcomings exist with known diesel engines. In diesel engines with indirect fuel injection (precombustion chamber), the engine's heat of compression during the compression stroke produces heat that begins to evaporate the air and fuel mixture in the cylinder. However, this compressing of the mixture increases the pressure. As a result, the increased pressure increases the boiling point of the fuel for evaporation. Evaporation continues slowly because these relationships are not linear, and just enough of the aromatics in the diesel fuel evaporate allowing it to fall within its flammability limits. The flash point temperature of the aromatics is low enough for the air and fuel mixture to auto-ignite, which results in a flame front. This flame front ignites more of the fuel mixture during the combustion process; however, it has the same effect of increasing the boiling point of the fuel so its critical temperature is never reached. Therefore, the remaining liquid fuel droplets do not evaporate before or during combustion.
Diesel engines with direct-injection (DI) have even greater fuel vaporization problems. In a diesel engine with DI high turbulence combustion chambers, the fuel spray pattern elongates in response to air flow. The smaller fuel droplets concentrate on the leading (lower) edge of the spray pattern while the larger and heavier droplets remain clustered about the core.
Ignition begins as a series of small bursts at the interface between the fuel spray and cylinder air, where there is surplus of oxygen. The bursts combine into flame fronts that progressively move into the fuel-soaked core of the pattern. Every normal combustion event in a diesel engine begins under oxygen-rich conditions and concludes under oxygen-lean conditions. This variability in fuel/air ratios is a special burden of the diesel engine. In addition, diesel engines operate under a fairly wide range of loads and speeds. Air turbulence, duration of the expansion stroke (power), and cylinder temperature vary with the operating mode.
Hydrocarbons survive their passage through the cylinder when the mixture is either too lean or too rich to burn. Excessively lean mixtures are caused by fuel droplets that break free of spray plume and diffuse throughout the combustion chamber. The resulting fuel mixture does not support combustion, and the raw fuel exists through the exhaust. This phenomenon often occurs under light loads and at low engine speeds, which causes high hydrocarbon emission spikes during idle. Hydrocarbon emissions are also generated when the flame is quenched by too rapid infusion of air or by contact with the relatively cool cylinder walls.
Particulate Matter (PM) in high concentrations that accompany diesel acceleration and cold starts can be seen as black smoke. The hydrocarbon component of PM, referred to as soluble organic fraction (SOF), consists of combustion by-products, lube oil and unburned fuel. Soot, the SOF carrier, forms in the oxygen-poor (rich fuel mixture) region on the trailing edge of the fuel plume. Oxides of nitrogen (NOx) are created in the high-temperature, oxygen-rich combustion (fuel-lean mixture) that occurs on the leading edge of the spray plume. Most soot forms early in the combustion process when fuel accumulates during the ignition lag period, then burns at extremely high temperatures to form NOx.
When the cylinder pressure falls due to the descent of the piston while on the power stroke, the fuel droplets that were not evaporated earlier now evaporate due to a lower boiling point and higher cylinder temperature. These evaporated fuel droplets now burn, but they burn too late into the crankshaft angle for producing power. Thus, less power, high emission levels, and high exhaust gas temperatures result.
The heat of combustion for diesel fuel is 500 to 550.degree. F. (260 to 288.degree. C.) above ambient. Convention diesel exhaust gas temperatures are 1,100 to 1,300.degree. F. (593 to 704.degree. C.). As with a gasoline engine, this temperature difference (heat energy) between the diesel exhaust gas temperature and the heat of combustion is totally wasted as excessive exhaust gas temperature. Thus, the engine's cooling system must be enlarged to dissipate the higher exhaust gas temperatures due to the increased temperature differential around the exhaust side of the combustion chambers and exhaust ports. This wasted heat energy is dissipated to the atmosphere through the vehicle's radiator, and an equal amount of wasted heat energy is dissipated through the vehicle's exhaust pipes as excessively high exhaust gas temperatures.
The present invention addresses some of the above mentioned, and other, shortcomings associated with the prior art.